The present invention relates to a process for efficiently producing uranium oxides, and more particularly, to a process for effectively removing metallic impurities contained in a starting material of a yellow cake to produce uranium oxides with a high purity.
The term "yellow cake" used herein means a uranium oxide concentrate that results from milling uranium ore. It typically contains about 80 to 90% U.sub.3 O.sub.8.
Various processes for refining uranium have been proposed. Among them, a process wherein a uranium oxide is produced as an intermediate is employed widely in many countries. When a yellow cake used as a starting material in this process contains metallic impurities such as iron, copper, molybdenum, vanadium, aluminum, calcium, magnesium, sodium, potassium and the like, the purity of the resulting uranium oxide or uranium hexafluoride obtained by further refining thereof is reduced. Therefore, these metallic impurities must be removed.
These metallic impurities have conventionally been removed by solvent extraction in a tributyl phosphate/nitric acid system. In this process, the yellow cake containing the impurities is dissolved in nitric acid and then the impurities are removed by extraction with tributyl phosphate (TBP) solvent. Then a solution of uranyl nitrate UO.sub.2 (NO.sub.3).sub.2 thus obtained is concentrated by evaporation to precipitate uranyl nitrate hydrate (UNH process) or, alternatively, ammonia is added to the uranyl nitrate solution to crystallize ammonium diuranate (NH.sub.4).sub.2 U.sub.2 O.sub.7.2H.sub.2 O (ADU process). The uranyl nitrate hydrate (UNH) or ammonium diuranate (ADU) is thermally decomposed by roasting it at a temperature of 500.degree. C. or higher to form uranium oxide and to recover nitric acid or ammonia.
The above-mentioned solvent extraction process is not preferred from the viewpoint of environmental pollution, since a waste liquor containing nitrogen and phosphorus is formed in a large amount because nitric acid and tributyl phosphate are used. Further, the materials forming apparatus employed in the process are limited, since nitric acid is used. In addition, this process is economically disadvantageous, since the solvent extraction is conducted in multiple steps with a mixer-settler and, therefore, a large apparatus and a large area for the installation thereof are required. Another problem is that an explosion-proof apparatus is necessitated, since the solvent is easily inflammable.